1. Field of the Invention
The present invention relates to a display device of video images or picture images typified by a liquid crystal display device, and a production method thereof.
2. Related Background Art
FIGS. 11A to 11E illustrate production processes of an active matrix substrate used in an active matrix type liquid crystal display device as an example of a conventional display device. The production processes are described by reference to the drawings. First of all, an n-type silicon semiconductor substrate 1 is partially thermally oxidized to form LOCOS (local oxidation of silicon) oxide films 2. The LOCOS oxide films 2 are used as a mask to ion-implant impurities, thereby forming PWLs 3. This substrate 1 is thermally oxidized again to form gate oxide films 4 (FIG. 11A).
After gate electrodes 5 composed of an n-type polysilicon doped with impurities are then formed, impurities are ion-implanted in the whole surface of the substrate 1 to form NLDs 6 which are n-type impurity regions. Impurities are subsequently ion-implanted using a patterned photoresist as a mask to form source and drain regions 7 and 7' (FIG. 11B).
A-PSG (phospho-silicate glass; phosphate glass) layer 8, which is an interlayer film, is formed on the whole surface of the substrate 1. After contact holes are patterned at portions of the PSG layer 8, which are situated right over the source and drain regions 7 and 7', and an Al film is formed on the PSG layer 8 by sputtering, the Al film is patterned to form Al electrodes 9 (FIG. 11C).
A PSG layer 11, which is an interlayer film, is formed in a thickness of about 10,000 .ANG. on the whole surface of the substrate 1, and through-holes are patterned (FIG. 11D).
A film of a metallic material such as Al, Ti, Ta or W, or a compound thereof is formed in a thickness of about 5,000 .ANG. on the surface of the PSG layer 11 on the substrate 1 by sputtering or the like, thereby forming pixel electrodes 12 by patterning (FIG. 11E).
Besides the active matrix substrate using such a silicon substrate, an example of an active matrix substrate using an insulating substrate is illustrated in FIG. 12.
FIG. 12 is a cross-sectional view of an active matrix substrate for a liquid crystal display device described in U.S. Pat. No. 4,024,626. In the drawing, reference numerals 120, 131, 132, 133, 135 and 136 indicate an insulating substrate, a gate electrode, a potential-fixing electrode, a pixel capacitance electrode, an interlayer film and a pixel electrode, respectively.
In the display device illustrated in FIGS. 11A to 11E, the surface of a pixel electrode 13 is influenced by difference in level of the underlying layer, and hence becomes uneven as illustrated in FIG. 11E. Besides, as apparent from FIG. 11E, this substrate requires cutting a groove having the same depth as the film thickness of the pixel electrode 13 between the individual pixel electrodes so as to insulate them from each other.
When light strikes on this pixel electrode 13, the incident light is scattered all around due to the irregularities of its surface, and so the reflection efficiency of light becomes very low. Besides, the surface irregularities form the cause of a failure in orientation in an orientation film-rubbing step of a packaging process of a liquid crystal. As a result, a failure to orientate the liquid crystal is caused, and the quality of images displayed is deteriorated due to the lowering of contrast.
Besides, since the portions corresponding to the grooves between the individual pixel electrodes are not rubbed, the cause of a failure in the orientation of the liquid crystal is formed, and at the same time, a lateral electric field is generated between the pixel electrodes conjointly with the surface irregularities, which electric field causes bright lines. The contrast of images displayed is markedly deteriorated by the occurrence of the bright lines, resulting in the reduction of image quality.
Even in the active matrix substrate illustrated in FIG. 12, the pixel electrodes 136 are insulated from each other. Therefore, grooves are cut in the same manner as illustrated in FIG. 11E, and the substrates is hence concerned about the fact that the same disadvantages as described above are brought about.